This invention is concerned with improving the catalytic performance of petroleum upgrading units operated under conditions which tend to cause maldistribution of the charge to the upgrading reactor. This invention is particularly concerned with hydrotreating processes such as catalytic lube dewaxing processes (LDW) wherein product formed in the dewaxing process is fed directly back into the reactor to improve catalyst performance, produce a higher yield of product and a product of higher viscosity index. Also highly suitable for adaptation in accordance with the present invention are catalytic distillate dewaxing (DDW), catalytic hydrodesulfurization (CHD) and heavy oil desulfurization (HOD). Accordingly, the concept disclosed herein should be applicable to any two-phase trickle-bed hydrotreating process.
The catalytic upgrading of petroleum base stock is well known in the art. U.S. Pat. No. 4,191,636 relates to a process for converting heavy hydrocarbon oil containing asphaltenes and heavy metals. Disclosed therein are U.S. Pat. No. 2,559,285, and U.S. Pat. No. 4,062,758 which propose to recycle a part of the liquid reaction product after being separated as a heavy fraction.
U.S. Pat. No. 4,180,453 is directed to a process for treating a hydrocarbon charge (a mixture of a hydrocarbon charge with a recycle fraction) in the presence of a bifunctional catalyst and hydrotreating the resultant effluent to separate nitrogen and sulfur therefrom and subjecting at least a portion of the effluent from the hydrotreatment step to a stream cracking step, recovering a gas oil fraction which is subjected to hydrogenation thereby forming said recycle fraction. Preferably a monofunctional catalyst is used prior to the bifunctional catalyst.
Catalytic dewaxing per se is also known and described in reissue patent, U.S. Pat. No. Re. 28,398 to Chen et al. The use of zeolites is also well known in the art. U.S. Pat. No. 3,778,501 discloses preparation of aromatics by contacting olefins over ZSM-5 type catalysts. U.S. Pat. No. 3,756,942 discloses an aromatization process involving contact of various hydrocarbyl feeds over ZSM-5 type catalysts. U.S. Pat. No. 4,483,760 discloses a process for catalytically dewaxing a middle distillate over a ZSM-5 type catalyst.
The prior art dewaxing processes wherein the catalyst used is of the ZSM-5 type is believed to operate at least in part by selectively cracking the waxy normal and singly methyl-substituted paraffins to form lower molecular weight olefins and paraffins. Regardless of mechanism, such dewaxing results in the formation of a by-product hydrocarbon fraction that has a lower average molecular weight that the charge and a substantial content of olefins. It is thus apparent that dewaxing of various oils and fuels is necessary to facilitate production of the desired dewaxed product in higher yield and improved quality, for example, higher V.I.